Digital audio: "audio that has been converted into a series of ones and zeros that can be understood by a computer. Digitized sound is easier to reproduce and manipulate without loss in quality than analog."
Sampling rate: "number of times per second the program converting analog to digital 'listens' to the analog signal." Typical baseline sample rate is 44.1KHz, though higher rates are becoming more common (48KHz, 96KHz, and 192KHz).
Bit depth: "level of detail at which a computer samples analog audio to create digital audio." 16-bit is considered standard for CDs; 24-bit sampling is more common and 32 bit float is good for better quality.
Bit rate: "number of kilobits per second of data in your audio file." Determines the size and quality of the resulting MP3. The standard bitrate is 128 kbps (take up about 1 Mb per 1 minute of sound), but 256 kbps is becoming more common (e.g. iTunes Plus).
Gain: amount of increase in audio signal strength, often expressed in dB
-Definitions from Industrial Audio Software and audio recording terms.